The present invention relates to an intake port structure for an internal combustion engine having a plurality of cylinders.
There are known internal combustion engines having an intake system composed of a primary intake passage for supplying an air-fuel mixture to an engine cylinder when the engine operates under a full range of loads and a secondary intake passage for supplying an additional air-fuel mixture to the engine cylinder when the engine operates under relatively high loads. Where such an intake system is incorporated in an internal combustion engine having a plurality of cylinders, the air-fuel mixture tends to flow back from the primary intake passages coupled to those cylinders having closed intake valves as leakage into their secondary intake passages, and then into a secondary intake passage connected to a cylinder with its intake valve open. Such an air-fuel mixture leakage then flows into the latter cylinder, lowering the speed at which an air-fuel mixture flows from the primary intake passage into the cylinder, and hence weakening the air-fuel mixture swirl created within the combustion chamber.
Prior efforts to prevent such an air-fuel mixture leakage through the secondary intake passages include check valves or throttle valves mounted in the secondary intake passages, respectively. Use of check valves, however, increases resistance to the flow of the air-fuel mixture through the secondary intake passages. The throttle valves are also disadvantageous in that the entire construction becomes quite complex.